Weakening of the Gram-negative bacterial outer membrane: A tool for increasing microbiological safety: Dissertation

Research output: ThesisDissertationCollection of Articles


Gram-negative bacteria are harmful in various surroundings. In the food industy their metabolites are a potential cause of spoilage and this group also includes many severe or potential pathogens. Due to their ability to produce biofilms Gram-negative bacteria also cause problems in many industrial processes as well as in clinical surroundings. Control of Gram-negative bacteria is hampered by the outer membrane (OM) in the outermost layer of the cells. This layer is an intrinsic barrier for many hydrophobic agents and macromolecules. Permeabilizers are compounds that weaken the OM and can thus increase the activity of antimicrobials by facilitating entry into the cells of external substances capable of inhibiting or destroying cellular funcions. The work described in this thesis demonstrates that lactic acid acts as a permeabilizer and destabilizes the OM of Gram-negative bacteria. In addition, organic acids present in berries, i.e. malic, sorbic and benzoic acids, were shown to weaken the OM of Gram-negative bacteria. Microbial colonic degradation products of plant-derived phenolic compounds (e.g. 3,4-dihydroxyphenylacetic acid, 3-hydroxyphenylacetic acid, 3,4-dihydroxyphenylpropionic acid, 4 hydroxyphenylpropionic acid and 3 hydroxyphenylpropionic acid) efficiently destabilized OM of Salmonella. The studies increase our understanding of the mechanism of action of the classical chelator, ethylenediaminetetraacetic acid (EDTA). In addition, the results indicate that the biocidic activity of benzalkonium chloride against Pseudomonas can be increased by combined use with polyethylenimine (PEI). In addition to PEI, several other potential permeabilizers, such as succimer, were shown to destabilize the OM of Gram-negative bacteria. Furthermore, combination of the results obtained from various permeability assays (e.g. uptake of a hydrophobic probe, sensitization to hydrophobic antibiotics and detergents, release of lipopolysaccharide (LPS) and LPS-specific fatty acids) and atomic force microscopy (AFM) image results increases our knowledge of the action of permeabilizers.
Original languageEnglish
QualificationDoctor Degree
Awarding Institution
  • University of Helsinki
  • Helander, Ilkka, Supervisor, External person
  • Saarela, Maria, Supervisor
Award date1 Jun 2007
Place of PublicationEspoo
Print ISBNs978-951-38-7014-0
Electronic ISBNs978-951-38-7015-7
Publication statusPublished - 2007
MoE publication typeG5 Doctoral dissertation (article)



  • Gram-negative bacteria
  • bacterial membranes
  • outer membrane
  • permeabilization
  • chelators
  • destabilizers
  • organic acids
  • EDTA
  • ethylenediaminetetraacetic acid
  • polyethylenimine

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